The present invention relates to a double-shaft vacuum pump with pistons rotating without contact in a pump chamber and with two synchronization gears located in a side chamber.
A typical example of a double-shaft vacuum pump is a Root's pump, in which two symmetrically designed pistons rotate in a pump housing without contact with each other or with the housing. The two pistons have approximately "figure-8" cross sections. Their movement is synchronized by gearing normally consisting of two synchronization gears. The gearing is housed in a side chamber which is separated from the pump chamber of the Root's pump by an end shield. The shafts extend through the end shield and generally are supported by ball bearings. The synchronization gears are fixed on the shaft ends extending into the side chamber. Normally, lubrication of the synchronization gears and bearings is done by oil present in the side chamber, the oil being swirled around by means of a centrifugal disk. Furthermore, Northey pumps, helical compressors, etc., which can also be used as vacuum pumps, belong to the double-shaft group of machines.
Double-shaft vacuum pumps of the type mentioned above have the advantage that there is no friction in the pump chamber, so that the pump chamber can be kept free of lubricants. For this reason they are particularly suitable for use with pumping media which must be kept free of carbon-containing contaminants. An important prerequisite for this is effective seals at the feed-through of the shafts into the oil-containing side chamber. Normally, piston ring seals are used as seals between the side chamber and the pump chamber. These as well as other known pump chamber seals, for example rotary shaft seals or rotating mechanical seals, do not present satisfactory solutions for the double-shaft vacuum pump, i.e. an assured seal between the pump chamber and the side chamber with the gearing so as to continuously assure an absolutely dry (i.e. oil-free) pump chamber.
It is known from German Laid-open Patent Application DE-OS 3,344,953 to evacuate the side chambers of a Root's pump in order to thereby maintain a slight flow of the pumping medium, in the area of the shaft seals, directed towards the side chambers. Such measures are expensive. Furthermore, such measures cannot be used in connection with corrosive or toxic pumping media.